Compensated impedance calibration circuit

ABSTRACT

Aspects of the invention provide for compensating impedance calibration circuits. In one embodiment, a compensated impedance calibration circuit, includes: a variable resistor network including a tunable resistor and a fixed resistor; and an external resistance network including a target external precision resistor and a parasitic distribution resistance; wherein a resistance of the variable resistor network is proportional to a resistance of the external resistance network, such that a ratio of an output voltage of the variable resistor network to a power supply voltage is constant.

FIELD OF THE INVENTION

The disclosure relates generally to calibration circuits, and moreparticularly, to compensated impedance calibration circuits, and arelated method.

BACKGROUND

Traditional input/output (I/O) driver impedance calibration circuitsnormally experience parasitic external resistance in the pin and packagedistribution network. This parasitic resistance incorporates aresistance on each side of an external precision target resistor.However, this parasitic resistance is usually limited to a small value.

However, for a 1:1 calibration architecture with a low impedance and atight tolerance, the parasitic external resistance variation may consumea large amount of the tolerance budget. For example, if the impedance is40 ohms, and there is a tolerance of 10% (±4 ohms), a small resistanceof 1.5 ohms on either side of the external precision target resistor(total of 3 ohms) takes up 3.8% of the 10% tolerance.

BRIEF SUMMARY

Aspects of the invention provide for compensating impedance calibrationcircuits. In one embodiment, a compensated impedance calibrationcircuit, comprising: a variable resistor network including a tunableresistor and a fixed resistor; and an external resistance networkincluding a target external precision resistor and a parasiticdistribution resistance; wherein a resistance of the variable resistornetwork is proportional to a resistance of the external resistancenetwork, such that a ratio of an output voltage of the variable resistornetwork to a power supply voltage is constant.

A first aspect of the disclosure provides a compensated impedancecalibration circuit, comprising: a variable resistor network including atunable resistor and a fixed resistor; and an external resistancenetwork including a target external precision resistor and a parasiticdistribution resistance; wherein a resistance of the variable resistornetwork is proportional to a resistance of the external resistancenetwork, such that a ratio of an output voltage of the variable resistornetwork to a power supply voltage is constant.

A second aspect of the disclosure provides a method of designing acompensated calibration circuit, comprising: determining a minimum and amaximum resistance for a parasitic distribution resistance; determininga value of a precision target resistor; calculating a minimum and amaximum resistance for an external resistance network, based on theminimum and the maximum parasitic distribution resistance and theprecision target resistor; and calculating a minimum and a maximumresistance for a fixed resistor, based on the minimum and the maximumresistance for the external resistance network and a chosen tunableresistor.

A third aspect of the disclosure provides a computer-implemented methodof designing a compensated calibration circuit, comprising: determininga minimum and a maximum resistance for a parasitic distributionresistance; determining a value of a precision target resistor;calculating a minimum and a maximum resistance for an externalresistance network, based on the minimum and the maximum parasiticdistribution resistance and the precision target resistor; andcalculating a minimum and a maximum resistance for a fixed resistor,based on the minimum and the maximum resistance for the externalresistance network and a chosen tunable resistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the disclosurewill be better understood by reading the following more particulardescription of the disclosure in conjunction with the accompanyingdrawings.

FIG. 1 shows a circuit diagram of a conventional impedance calibrationcircuit.

FIG. 2 shows a circuit diagram of a conventional impedance calibrationcircuit.

FIG. 3 shows a circuit diagram of a compensated impedance calibrationcircuit according to embodiments of the invention.

FIG. 4 shows a graph of comparing the error due to distributionresistance in a traditional and a compensated calibration circuit.

FIG. 5 is an illustrative environment for designing a compensatedcalibration circuit according to embodiments of the invention.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the disclosure. The drawings are intended to depict only typicalembodiments of the disclosure, and therefore should not be considered aslimiting the scope of the disclosure. In the drawings, like numberingrepresents like elements.

DETAILED DESCRIPTION

The disclosure relates generally to calibration circuits, and moreparticularly, to compensated impedance calibration circuits, and arelated method.

Traditional input/output (I/O) driver impedance calibration circuitsnormally experience parasitic external resistance in the pin and packagedistribution network. This parasitic resistance incorporates aresistance on each side of an external precision target resistor.However, this parasitic resistance is usually limited to a small value.

However, for a 1:1 calibration architecture with a low impedance and atight tolerance, the parasitic external resistance variation may consumea large amount of the tolerance budget. For example, if the impedance is40 ohms, and there is a tolerance of 10% (±4 ohms), a small resistanceof 1.5 ohms on either side of the external precision target resistor(total of 3 ohms) takes up 3.8% of the 10% tolerance.

Turning now to the figures, in FIG. 1, a circuit diagram of aconventional calibration circuit 1 is shown. The calibration circuit 1includes a chip side portion 10 and an external resistance network 20.In the external resistance network 20, there is a target precisionresistor 22 and a parasitic distribution resistance 24. The parasiticdistribution resistance 24 is formed by two resistors 26, 28 that are oneither side of the target precision resistor 22 (i.e., in series withthe target precision resistor 22).

Turning now to FIG. 2, a more detailed circuit diagram of a conventionalcalibrated circuit 1 is shown. The circuit 1 includes a voltagegenerator 29, a comparator 40, and control logic 50. In conventionaloperation, a ratio of the target precision resistor 22 to a tunableresistor 30 within the chip 10 sets an internal voltage (“PADR”). Incomparing (via comparator 40) the internal voltage (“PADR”) to areference voltage (“VREF”) that is generated by voltage generator 29,the value of the tunable resistor 30 is tuned, via control logic 50 toachieve the correct output resistance. The voltage generator 29 maycomprise, for example, a voltage divider, as known in the art, betweentwo resistors 32A, 32B.

However, when the target precision resistor 22 in the externalresistance network 20 is small (e.g., less than 40 ohms), the parasiticdistribution resistance 24 provided by resistors 26, 28 consumes a largeportion of the tolerance budget.

Aspects of the invention provide for compensating impedance calibrationcircuits. In one embodiment, a compensated impedance calibration circuitis provided. The circuit includes: a variable resistor network includinga tunable resistor and a fixed resistor; and an external resistancenetwork including a target external precision resistor and a parasiticdistribution resistance; wherein a resistance of the variable resistornetwork is proportional to a resistance of the external resistancenetwork, such that a ratio of an output voltage of the variable resistornetwork to a power supply voltage is constant.

Turning now to FIG. 3, a circuit diagram of a compensated calibrationcircuit 100 according to aspects of the invention is shown. Similar tothe calibration circuit 1 shown in FIGS. 1 and 2, there is an externalresistance network 20 including a target external precision resistor 22and a parasitic distribution resistance 24 (resistors 26, 28). However,compensated calibration circuit 100 includes a variable resistor network110 that includes a tunable resistor 130 and also a fixed resistor 140.As will be described herein, the fixed resistor 140 is “fixed” in termsof a value, as compared to tunable resistor 130, which may be tuned.However, the fixed resistor 140, as will be described later, is a“pluggable” resistor. That is, the resistance value of the fixedresistor 140 may be a range of discrete values, depending on theexternal resistance network 20. The range of discrete values for thefixed resistor 140 will be such that the variable resistor network 110is proportional to the external resistance network 20. Therefore, theratio of the output voltage (“PADR”) of the variable resistor network110 to a power supply voltage (“DVDD”) is constant. The followingequation represents the ratio:

$\frac{PADR}{DVDD} = \frac{R_{{ext}{({tgt})}} + R_{dist}}{R_{tuned} + R_{{fix}{({plg})}} + R_{{ext}{({tgt})}} + R_{dist}}$where R_(ext(tgt)) is the value of the target external precisionresistor 22, R_(dist) is the value of the parasitic distributionresistance 24 (sum of resistors 26, 28), R_(tuned) is the value of thetunable resistor 130, and R_(fix(plg)) is the value of the fixedresistor 140.

A method of designing the compensated calibration circuit 100 includesdetermining the minimum and maximum values for the fixed resistor 140,based on the minimum and maximum values for the external resistancenetwork 20 and a chosen tunable resistor 130. The following methodassumes a resistance of 40 ohms (Ω), however, this is for exemplarypurposes only, and other small resistances are applicable.

First, a minimum and maximum resistance for the parasitic distributionresistance 24 (R_(dist)) is determined. For example, the minimumresistance may be 0.25Ω and the maximum resistance may be 3.25Ω. Basedon the minimum and maximum parasitic distribution resistances, and theprecision target resistor 22 (R_(ext(tgt))), the minimum and maximumvalues for the external resistance network 20 are calculated. Forexample, the minimum resistance of the external resistance network 20may be 40.25Ω and the maximum resistance of the external resistancenetwork 20 may be 43.25Ω (i.e., R_(dist)+R_(ext(tgt)) for the minimumR_(dist) and the maximum R_(dist)).

For a chosen tunable resistor 130, the minimum and maximum values forthe fixed resistor 140 are calculated. For example, the tunable resistor130 may be 31.5Ω The total resistance of the external resistance network20 is equal to the total resistance of the variable resistor network100. Therefore, following the example, the minimum value for the fixedresistor 140 would be 8.75Ω (i.e., subtract the tunable resistor 130from the total minimum value for the external resistance network 20).That is, 40.25Ω−31.5Ω.

For the maximum value for the fixed resistor 140, the same equationwould apply. However, the total maximum value for the externalresistance network 20 is 43.25Ω. Therefore, the maximum fixed resistor140 value is 11.25Ω (e.g, 43.25Ω−31.5Ω).

The tunable resistor 130 may be chosen, based on a comparison of theoutput voltage of the variable resistor network 100 (“PADR”) to thereference voltage (“VREF”) via comparator 40 (FIG. 2). Further, once themaximum and minimum values for the fixed resistor 140 are chosen, anumber of values that can be selected for fixed resistor 140 for adesired tolerance are determined. For example, in the example above, ifthe range is from 8.75Ω to 11.25Ω for the fixed resistor 140, if six (6)resistors can be selected for fixed resistor 140, there would be a stepof 0.5Ω per resistor. Therefore, the resistance for fixed resistor 140may be 8.75Ω, 9.25Ω, 9.75Ω, 10.25Ω, 10.75Ω, and 11.25Ω.

Turning now to FIG. 4, a graph comparing the error due to distributionresistance in a traditional and a compensated calibration circuit, suchas the circuit 100 shown in FIG. 3, is shown. As the distributionresistance increases, for a traditional calibration circuit, the errorincreases linearly. However, with a compensated calibration circuit, theerror range is reduced significantly. Therefore, the parasiticdistribution resistance 24 (i.e., from resistors 26, 28) does not takeup a large portion of the tolerance budget.

FIG. 5 shows an illustrative environment 300 for designing a compensatedcalibration circuit according to an embodiment. To this extent,environment 300 includes a computer system 200 that can perform aprocess described above in order to design a compensated calibrationcircuit. In particular, computer system 200 is shown including acompensation program 230, which makes computer system 200 operable todesign a compensated calibration circuit by performing a processdescribed above.

That is, when a chosen tunable resistor 202, precision target resistor204, and the minimum and maximum resistance for the parasiticdistribution resistor 206 is determined, the minimum and maximum valuesfor the fixed resistor 210 may determined, using the method describedabove, with compensation program 230.

Computer system 200 is shown including a processing component 222 (e.g.,one or more processors), a storage component 224 (e.g., a storagehierarchy), an input/output (I/O) component 226 (e.g., one or more I/Ointerfaces and/or devices), and a communications pathway 228. Ingeneral, processing component 222 executes program code, such ascompensation program 230, which is at least partially fixed in storagecomponent 224. While executing program code, processing component 222can process data, which can result in reading and/or writing transformeddata from/to storage component 224 and/or I/O component 226 for furtherprocessing. Pathway 228 provides a communications link between each ofthe components in computer system 200.

In any event, computer system 200 can comprise one or more generalpurpose computing articles of manufacture (e.g., computing devices)capable of executing program code, such as compensation program 230,installed thereon. As used herein, it is understood that “program code”means any collection of instructions, in any language, code or notation,that cause a computing device having an information processingcapability to perform a particular action either directly or after anycombination of the following: (a) conversion to another language, codeor notation; (b) reproduction in a different material form; and/or (c)decompression. To this extent, compensation program 230 can be embodiedas any combination of system software and/or application software.

Further, compensation program 230 can be implemented using a set ofmodules 232. In this case, a module 232 can enable computer system 200to perform a set of tasks used by compensation program 230, and can beseparately developed and/or implemented apart from other portions ofcompensation program 230. As used herein, the term “component” means anyconfiguration of hardware, with or without software, which implementsthe functionality described in conjunction therewith using any solution,while the term “module” means program code that enables a computersystem 200 to implement the actions described in conjunction therewithusing any solution. When fixed in a storage component 224 of a computersystem 200 that includes a processing component 222, a module is asubstantial portion of a component that implements the actions.Regardless, it is understood that two or more components, modules,and/or systems may share some/all of their respective hardware and/orsoftware. Further, it is understood that some of the functionalitydiscussed herein may not be implemented or additional functionality maybe included as part of computer system 200.

When computer system 200 comprises multiple computing devices, eachcomputing device can have only a portion of compensation program 230fixed thereon (e.g., one or more modules 232). However, it is understoodthat computer system 200 and compensation program 230 are onlyrepresentative of various possible equivalent computer systems that mayperform a process described herein. To this extent, in otherembodiments, the functionality provided by computer system 200 andcompensation program 230 can be at least partially implemented by one ormore computing devices that include any combination of general and/orspecific purpose hardware with or without program code. In eachembodiment, the hardware and program code, if included, can be createdusing standard engineering and programming techniques, respectively.

Regardless, when computer system 200 includes multiple computingdevices, the computing devices can communicate over any type ofcommunications link. Further, while performing a process describedherein, computer system 200 can communicate with one or more othercomputer systems using any type of communications link. In either case,the communications link can comprise any combination of various types ofwired and/or wireless links; comprise any combination of one or moretypes of networks; and/or utilize any combination of various types oftransmission techniques and protocols.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A compensated impedance calibration circuit,comprising: a variable resistor network including a tunable resistor anda fixed resistor; and an external resistance network including a targetexternal precision resistor and a parasitic distribution resistance,wherein a resistance of the variable resistor network is proportional toa resistance of the external resistance network, such that a ratio of anoutput voltage of the variable resistor network to a power supplyvoltage is constant, wherein a value for the fixed resistor is selectedfrom a range of discrete values, the range of discrete values including:a minimum value represented by (R_(ext(tgt))+R_(distmin))−R_(tuned); anda maximum value represented by (R_(ext(tgt))+R_(distmax))−R_(tuned),wherein R_(ext(tgt)) is a resistance value of the target externalprecision resistor, R_(distmin) is a minimum resistance value of theparasitic distribution resistance, R_(distmax) is a maximum resistancevalue of the parasitic distribution resistance, and R_(tuned) is aresistance value of the tunable resistor.
 2. The compensated impedancecalibration circuit of claim 1, further comprising a voltage referencegenerator for generating a reference voltage.
 3. The compensatedimpedance calibration circuit of claim 2, further comprising acomparator for comparing the reference voltage to the output voltage ofthe variable resistor network.
 4. The compensated impedance calibrationcircuit of claim 3, further comprising control logic for receiving anoutput of the comparator and for tuning the variable resistor network,such that the output voltage of the variable resistor network is equalto the reference voltage.
 5. The compensated impedance calibrationcircuit of claim 3, further comprising control logic for receiving anoutput of the comparator and for tuning the variable resistor network,such that the output voltage of the variable resistor network is equalto the reference voltage.
 6. The compensated impedance calibrationcircuit of claim 1, wherein the fixed resistor is a pluggable on-chipresistor.
 7. The compensated impedance calibration circuit of claim 1,wherein the value for the fixed resistor in the range of discrete valuesdepends upon a resistance of the external resistance network.
 8. Thecompensated impedance calibration circuit of claim 1, wherein the ratioof the output voltage of the variable resistor network (PADR) to thepower supply voltage (DVDD) is represented by:${\frac{PADR}{DVDD} = \frac{R_{{ext}{({tgt})}} + R_{dist}}{R_{tuned} + R_{{fix}{({plg})}} + R_{{ext}{({tgt})}} + R_{dist}}},$wherein R_(dist) is a resistance value of the parasitic distributionresistance.
 9. A compensated impedance calibration circuit, comprising:a variable resistor network including a tunable resistor and a fixedresistor; and an external resistance network including a target externalprecision resistor and a parasitic distribution resistance, wherein atotal resistance of the variable resistor network is equal to a totalresistance of the external resistance network, such that a ratio of anoutput voltage of the variable resistor network to a power supplyvoltage is constant, wherein the ratio of the output voltage of thevariable resistor network (PADR) to the power supply voltage (DVDD) isrepresented by:${\frac{PADR}{DVDD} = \frac{R_{{ext}{({tgt})}} + R_{dist}}{R_{tuned} + R_{{fix}{({plg})}} + R_{{ext}{({tgt})}} + R_{dist}}},$wherein R_(ext(tgt)) is a resistance value of the target externalprecision resistor, R_(dist) is a resistance value of the parasiticdistribution resistance, R_(tuned) is a resistance value of the tunableresistor, and R_(fix(plg)) is a resistance value of the fixed resistor.10. The compensated impedance calibration circuit of claim 9, furthercomprising a voltage reference generator for generating a referencevoltage.
 11. The compensated impedance calibration circuit of claim 10,further comprising a comparator for comparing the reference voltage tothe output voltage of the variable resistor network.
 12. The compensatedimpedance calibration circuit of claim 9, wherein the fixed resistor isa pluggable on-chip resistor.
 13. The compensated impedance calibrationcircuit of claim 9, wherein the value for the fixed resistor in therange of discrete values depends upon a resistance of the externalresistance network.